There is only one line from the air pump, the pump has an opening with a grate on it that it sucks air from, then that air is pushed into the exhaust when the engine is cold, this enriches the exhaust with oxygen to assist the converters catalyst while the fuel mixture is rich. Mine sounds like a vacuum cleaner in the morning. I am not aware of air going the other way. If this is happening you need to check the bypass valve on top of the manifold as it may be defective and allowing exhaust into the pump, this will ruin the pump. The hot exhaust if it hits the pump will burn it out. Good luck!
Brian

__________________
Brian W. Heitman

www.MBCA.org/northtexas

Join MBCA!!!